One of the prior art magnetic tape guiding cylinder device is shown in FIG. 1. Reference numeral 1 designates an upper cylinder and 2 is a lower cylinder fixed to a base plate not shown by a suitable method. The lower cylinder 2 is formed along the circumferential surface thereof with a tape travel path 2a. A cylindrical member 2c is integrally formed with and penetrates through a bottom plate 2b of the lower cylinder 2. Two bearings 3 and 4 are fitted at both ends of the cylindrical member 2c. A drive shaft 5 is fixed to the upper cylinder 1. The upper cylinder 1 is rotatably supported by the lower cylinder 2 via the drive shaft 5 and the bearings 3 and 4. Magnetic heads 6a and 6b are fixed to the circumference of the upper cylinder 1. A magnetic tape T engages the tape travel path 2a and travels slightly obliquely about a half revolution of the circumference of the cylinders 1 and 2. The upper cylinder 1 rotates at a given speed responsively to the travel of the magnetic tape T in the same direction. So, if the upper cylinder 1 vibrates or deviates radially during the rotation, such vibration or deviation makes noises during recording or playback and damages the reproduced picture. Main factors causing such vibrations or deviations are inaccurate roundness of the upper cylinder 1, deviation of the rotation axle thereof, and vibration of rolling steel balls of the bearings 3 and 4. Therefore, it is important to precisely finish the upper and lower cylinders to keep an acceptable roundness of the upper cylinder 1 and of bearing mounting surfaces 2d and 2e (FIG. 2) along the inner periphery of the cylindrical member 2c.
FIG. 2 shows how to finish the lower cylinder 2. First, the outer periphery of the cylindrical member 2c is finished so as to have a sufficient smoothness. A three-way chuck 7 engages this finished outer periphery and unmovably supports the lower cylinder for succeeding finishing operations thereof. Thus, the bearing mounting surfaces 2d and 2e of the inner periphery of the cylindrical member 2c and the tape travel path 2a along the outer periphery of the lower cylinder 2 are cut and finished. For this purpose, the fastening force of the three-way chuck 7 must overcome the rotating tendency of the lower cylinder 2 produced by the cutting operation. This force will be substantially proportional to the contact area between the chuck 7 and the cylindrical member 2c, i.e. to the length L of the contacting surface of the chuck 7 and to the outer diameter of the cylindrical member 2c, or alternatively proportional to the fastening force of the chuck 7.
However, keeping a sufficiently large friction area does not meet a recent market demand of reducing the size and weight of such a video tape recorder.
To compensate an insufficient friction area, it is necessary to increase the fastening force of the chuck 7. The latter measure, however, produces another problem in the prior art. Namely, even if the chuck 7 contacts the central portion of the cylindrical member 2c, avoiding one end portion corresponding to the bearing mounting surface 2d, the directly engaged portion of the cylindrical member 2c is deformed due to a strong pressure from the chuck 7 and this deformation reaches the said end portion in question. So, even if an acceptable roundness of the bearing mounting surface 2d is created during the deformation, the roundness is distorted when the three-way chuck 7 is removed and the cylindrical member 2c recovers its original aspect. The distorted bearing mounting surface 2d distorts the roundness of an outer ring 3a of the bearing 3 as shown in FIG. 3. As a result, the steel balls 3b vibrate during their rolling movement and cause deterioration of the reproduced picture.